1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to a backlight unit of a display device employing DDAM (Divided Display Area Method) in which a display area is divided into a plurality of regions for operation, among Field Sequential (FS) driving methods, and to a liquid crystal display device using the backlight unit.
2. Discussion of the Related Art
CRT (Cathode Ray tube), one of general display devices, is mainly being used as a monitor for television (TV), measuring machine, information terminal, etc., but fails to cope positively with requests for miniaturization and lightness of electronics products due to the size and weight of the CRT itself.
Thus, CRT has a limitation in decreasing the weight and volume, which is contrary to a current tendency of the miniaturization and lightness of the electronic products. As a candidate anticipated to replace the CRT, there are liquid crystal display (LCD) using electro-optical effect, plasma display panel (PDP) using gas discharge, electro-luminescence display (ELD) device, and so on. Among these candidates, LCD is most actively being researched.
In order to replace the CRTs, LCDs are actively being developed because of their small size, light weight, and lower power consumption characteristics. Recently, LCDs advance to a degree to perform the roles as a flat panel display, and are being used as monitors for laptop computers, desktop computers, large-sized information displays, etc., so that demands for the LCDs continue to increase.
The driving principle of liquid crystal display (LCD) devices utilizes optical anisotropy and polarization properties of liquid crystal. Liquid crystal has the directionality in their molecules alignment due to its slender and long structure. Hence, it is possible to control the orientation of the liquid crystal molecule by artificially applying an electric field to the liquid crystal.
Accordingly, by arbitrarily controlling the arrangement direction of the liquid crystal molecules, the alignment of the liquid crystal molecules is changed, so that an incident light is refracted in the alignment direction of the liquid crystal molecules to thereby display image information.
In nowadays, active matrix LCD (AM-LCD) in which thin film transistors (TFTs) as switching elements and pixel electrodes connected to the TFTs are arranged in a matrix configuration attracts public attention owing to its superior resolution and moving picture displaying capability.
Hereinafter, there will be reviewed a general liquid crystal display device in which image is realized by the aforementioned driving principle. FIG. 1 is a schematic view of a general liquid crystal display.
Referring to FIG. 1, the general liquid crystal display device includes: (a) a liquid crystal panel having first and second transparent glass substrates 1 and 10 attached to each other with a predetermined space therebetween, and a liquid crystal layer 15 interposed between the first and second glass substrates 1 and 10; and (b) a backlight 16 arranged on a rear side of the first glass substrate 10, for supplying the liquid crystal panel with light.
Here, the first glass substrate 1 serving as a TFT array substrate is provided with a plurality of gate lines (not shown) arranged at a predetermined interval in one direction, a plurality of data lines (not shown) arranged at a predetermined interval in another direction perpendicular to the gate lines, a plurality of pixel electrodes 2 arranged in a matrix configuration on pixel regions defined by the gate lines and the data lines crossing each other, and a plurality of thin film transistors (T) 3 each for being switched by a signal of the corresponding gate line and transferring a signal of the corresponding data line to the corresponding pixel electrode.
The second glass substrate 10 serving as a color filter substrate is provided with a black matrix layer 11 for shutting the light of the region except for the pixel region, a color filter layer 12 including red, green and blue cells for transmitting the light of a specific wavelength band and absorbing the lights of the remaining wavelength bands, and a common electrode 14 for realizing an image.
The non-described reference 13 denotes an overcoat layer.
The first and second glass substrates 1 and 10 are attached to each other by a sealant having a predetermined liquid crystal inlet and are spaced apart from each other with a predetermined space by spacers.
In FIG. 1, a unit pixel region is shown on the first and second glass substrates 1 and 10 for the convenience of description.
The liquid crystal display as described above needs a separate light source, that is, the backlight 16 so as to display an image by controlling the amount of the light applied from the external side to the liquid crystal panel.
Hereinafter, a general backlight unit will be described.
FIG. 2 illustrates a general backlight unit. As shown in FIG. 2, the general backlight unit includes a fluorescent lamp 21, a light guide plate 22, a diffusion material 23, a reflection plate 24, a diffusion plate 25 and a prism sheet 26.
First, when a voltage is applied to the fluorescent lamp 21, residual electrons in the fluorescent lamp 21 move to anode. The moving residual electrons collide with argon (Ar) molecules and excite the argon to increase cations. Increased cations collide with cathode to emit secondary electrons.
The emitted secondary electrons flow in the fluorescent lamp 21 to start discharging. The discharged electrons collide with mercuric vapor to ionize the mercuric vapor so that ultraviolet and visible lights are emitted. The emitted ultraviolet excites the fluorescent material coated on the inner wall of the lamp to emit visible light.
The light guide plate 22 serves as a wave-guide that allows the light emitted from the fluorescent lamp 21 to be incident into the inside of the liquid crystal panel and thus facial light to be projected upwards, and is made of poly methyl meth acrylate (PMMA) resin with good light transmittance.
As factors related to incident light efficiency of the light guide plate 22, there are the thickness of the light guide plate 22, the diameter of the lamp 21, the distance between the light guide plate 22 and the lamp 22, and the shape of the lamp reflection plate 24.
As the light guide plate 22 of the backlight unit for an LCD, there are a print type light guide plate, a V-cut type light guide plate and a scattering light guide plate.
The diffusion material 23 is composed of SiO2 particles, PMMA and solvent. The above-mentioned SiO2 particles are used for light diffusion and have porous particle structure. PMMA is used to attach the SiO2 particles to the lower surface of the light guide plate 22.
The diffusion material 23 is coated on the lower surface of the light guide plate 22 in a dot shape, and the dot area is gradually increased to obtain a uniform surface light source at the upper portion of the light guide plate 22. In other words, the dot area per unit area is small at a location close to the fluorescent lamp 21 and the dot area per unit area is large at a location far from the fluorescent lamp 21. Various shapes of the dots can be used. If the ratios of dot area per unit area are the same, the same brightness can be obtained at the upper portion of the light guide plate 22 regardless of the shape of the dots.
The reflection plate 24 is arranged below the light guide plate 22 and allows the light projected from the fluorescent lamp 21 to be applied into the light guide plate 22.
The diffusion plate 25 is arranged above the light guide plate 22 such that uniform brightness is obtained according to viewing angles. The material of the diffusion plate 25 is PET or poly carbonate (PC) resin. The upper portion of the diffusion plate 25 is coated with a particle coating layer for diffusing light.
The prism sheet 26 is used to enhance the front brightness of the light transmitted through the upper portion of the diffusion plate 25. The above-mentioned prism sheet 26 transmits only the light of a predetermined angle and fully reflects the light of other angles internally. The reflected light returns to the lower portion of the prism sheet 26. The returned light as described above is reflected by the reflection plate 24 attached to the lower portion of the light guide plate 22.
The backlight unit configured as above is fixed to a mold frame, and the display unit such as the liquid crystal panel disposed on the upper surface of the backlight unit is protected by a top chassis. The top chassis and the mold frame are coupled with each other accommodating the backlight unit and the display unit therebetween.
However, the general liquid crystal display configured as above has the following problems.
First, the transmittance of the light of the color filter of a general LCD device is less than 33% at most, which corresponds to a large light loss. To enhance the brightness by compensating for the light loss due to the color filter, the backlight should be made brighter. However, such a solution causes increase in the power consumption by the backlight and thus by the LCD device.
Second, since the color filter of a general LCD device is very expensive compared with the other materials of the LCD device, the color filter raises the production cost of the LCD device.
An LCD device suggested to solve these problems of the LCD device is a field sequential LCD device that implemented full-color without any color filter. The backlight of the general LCD device supplies the liquid crystal panel with white light in a state that the backlight is always turned on, but the field sequential LCD device turns on the R, G, B light sources of the backlight unit sequentially with a predetermined interval for one frame to display a color image. This field sequential method was suggested in 1960s, but it was very difficult to implement it since the technologies for a liquid crystal mode having a high speed response time and a light source meeting the high speed liquid crystal mode have to follow the field sequential method.
However, the recent amazing advancement in the LCD technologies enables to suggest a field sequential (FS) LCD device using a ferroelectric liquid crystal (FLC) mode, an optical compensated birefringent (OCB) mode or a twisted nematic (TN) liquid crystal mode and an R, G, B backlight unit that can turn on at a high speed.
Particularly, the field sequential LCD device mainly uses the OCB mode as the liquid crystal mode. The OCB cell is formed in a bend structure by rubbing the facing surfaces of an upper substrate and a lower substrate in the same direction and applying a predetermined voltage. If a voltage is applied, the liquid crystal molecules move rapidly so that the time necessary for realignment of the liquid crystal molecules, that is, the response time, is very quick and less than about 5 m/sec. Thus, since the OCB mode liquid crystal cell is a high speed response characteristic and does not nearly leave residual images on a screen, it is very suitable for a field sequential LCD device.
FIG. 3 is a schematic cross-sectional view of a general field sequential LCD device. As shown in FIG. 3, the general field sequential LCD device includes an upper substrate 30, a lower substrate 35 that is an array substrate, a liquid crystal layer 38 interposed between the upper and lower substrates 30 and 35, and R, G, B three color backlight 39 for supplying light to the liquid crystal panel including the upper and lower substrates 30 and 35 and the liquid crystal layer 38.
The upper and lower substrates 30 and 35 are respectively provided with a common electrode 32 and a pixel electrode 36 to which a voltage is applied so as to drive the liquid crystal layer 38. A black matrix 31 for shutting the light of the region except for the pixel electrode 36 of the lower substrate 35 is formed between the upper substrate 30 and the common electrode 32. A thin film transistor (T) 37 connected electrically to the pixel electrode 36 and acting as a switching element is formed on the lower substrate 35 at the position corresponding to the black matrix 31 of the upper substrate 30. Although not shown in the drawings, the thin film transistor (T) 37 includes a gate electrode, source electrode and drain electrode. Reference numeral 40 indicates an overcoat layer. For the convenience of description, only a unit pixel region of the upper and lower substrates 30 and 35 is shown in FIG. 3.
The above-mentioned field sequential LCD device can be apparently distinguished from the general LCD device in that the field sequential LCD device does not need the color filter layer and since the R, G, B light sources of the backlight unit are separately lit.
Hereinafter, the backlight unit having R, G, B light sources is briefly referred to as an R, G, B backlight.
The R, G, B backlight 39 is driven by one inverter (not shown). Each color of the backlight 39 lights 60 times per second and accordingly the three colors of the backlight 39 light 180 times per second to cause residual image effect on eyes and mix three colors. The R, G, B backlight 39 lights 180 times every second but looks like lighting on continuously.
For example, if the R light source lights and then the B light source lights, violet is seen to human eyes due to the residual image effect. The R, G, B backlight applies such a phenomenon. In other words, since the field sequential LCD device does not have any color filter, it can overcome the problem of the general LCD device where the light transmittance is low and the entire brightness of the LCD device is lowered. Also, since full color can be realized with three color backlight, high brightness and high definition characteristics can be obtained and production costs can be saved due to the omission of expensive color filter. As such, the field sequential LCD device is suitable for the large-sized LCD device.
Further, the general LCD device is inferior to the CRT in price and definition as described above, but the field sequential LCD device can solve this problem.
As described above, since most of LCD devices are passive devices that control the light amount from the external side to display images, they necessarily need a separate light source, i.e., backlight unit. In general, the backlight units of the LCD device are classified into a direct type and an edge type according to the arrangement of lamps.
In the direct type (or flat) backlight unit, since lamps are arranged on a plane, the shape of lamps is shown on the liquid crystal panel. To this end, it is necessary to secure a sufficient distance between the lamp and the liquid crystal panel. Also, light scattering means should be arranged for a uniform distribution of light amount. So, the direct type backlight LCD has a limitation in making the LCD device slim.
As the liquid crystal panel size increases, the area of the light output surface of the backlight unit increases too. If the direct type backlight unit is large-sized and the light scattering means does not secure a sufficient thickness, the light output is not flat. For this reason, it is required that the light scattering means should have a sufficient thickness.
In the meanwhile, in the edge type backlight unit, the lamps are disposed on an edge of the light guide plate, and the light guide plate is used to disperse the light by an entire surface thereof. The edge type backlight unit is problematic in low brightness since the lamp is installed at a side and light has to pass through the light guide plate. To distribute the light intensity uniformly, sophisticated optical design technology and processing technology for the light guide plate are required.
Since the direct type backlight unit and the edge type backlight unit have their disadvantages, the direct type backlight unit is usually used for the LCD device the brightness of which is more important than its thickness. The edge type backlight unit is usually used for the LCD device for a notebook PC or a monitoring PC the thickness of which is more important than its brightness.
FIGS. 4A and 4B are cross-sectional views of different backlight units for the field sequential LCD device. Specifically, FIG. 4A illustrates the edge type backlight unit and FIG. 4B illustrates the direct type backlight unit.
The edge type R, G, B backlight 40 shown in FIG. 4A is provided with a series of R, G, B light sources on one side surface or both side surfaces of a liquid crystal panel 41, and is a lighting apparatus that receives light from the light guide plate and reflection plate (not shown) and diffuses the light. The edge type R, G, B backlight 40 usually uses cold cathode fluorescent lamp (CCPL) as a light source. Since the edge type R, G, B backlight 40 has thin, light and low power consumption characteristics type, it is suitable for a portable computer.
The direct type R, G, B backlight 45 as shown in FIG. 4B is provided with R, G, B light sources 46 arranged below a scattering plate 47. Light from the R, G, B light sources 46 is directly irradiated onto the entire surface of the liquid crystal panel 41. The R, G, B light sources 46 constitute a plurality of single units each having R, G, B light sources 46 arranged in series horizontally.
This direct type R, G, B backlight 45 is used for an image display device the brightness of which is important. However, since it is too thick and needs the scattering plate to maintain the uniformity of the brightness, its power consumption is high.
FIG. 5A shows a portion of an array substrate of an LCD device to illustrate the driving method of the field sequential LCD device.
As shown in FIG. 5A, in general, the lower substrate that is an array substrate of the LCD device is provided with a plurality gate lines 50 in the horizontal direction, a plurality of data lines 51 crossing the gate lines 50 perpendicularly, a plurality of thin film transistors T each formed at the position at which the corresponding gate line 50 and the corresponding data line 51 cross each other, and a plurality of pixel electrodes 52 each connected electrically to the corresponding thin film transistor T.
In the driving method of the general LCD device, an image signal is applied to the data line 51 and an electric pulse is applied to the gate line 50 by a scanning method. The LCD device is driven by applying a selective gate pulse voltage to the gate line 50. To improve display quality, this gate pulse voltage applying method is a linear sequence driving method in which a voltage is applied to a gate line by one line by a gate scanning input device and is sequentially applied to a next gate line by a gate scanning input device line by line. The gate pulse voltage is applied to all the gate lines 50 so that one frame is completed.
In other words, if the gate pulse voltage is applied to the n-th gate line, all the thin film transistors T connected to this gate line to which the gate pulse voltage is applied are concurrently turned on. An image signal on the data line is stored in a liquid crystal cell and a storage capacitor through this turn-on thin film transistor T.
Accordingly, the liquid crystal molecules in the liquid crystal cell are realigned according to the data image signal stored in the liquid crystal cell and the voltage of the image signal so that the backlight is transmitted through the liquid crystal cell to realize the desired image.
FIG. 5B is a time chart illustrating the driving method of a field sequential LCD device according to a related art. In the driving method of the field sequential LCD device, all the thin film transistors are scanned according to R, G, B light sources and the liquid crystal molecules are completely realigned to the light from each of the R, G, B backlight sources. In other words, for the entire driving regions, the backlight unit is configured to light once every backlight source for one frame.
This driving process should be performed within one period (f/3) for each backlight source (R, G, B) of the backlight unit. In other words, considering one backlight source as a standard, one period for each backlight source is as follows:f/3(55)=tTFT(56)+tLC(57)+tBL(58)                where f: frame frequency,        tTFT: scanning time of the entire thin film transistor,        tLC: response time of assigned liquid crystal, and        tBL: flash time of backlight.        
Here, when tBL (58) is set to be a fixed value and tTFT (56) increases according to the design condition of the LCD device, since the interval between frames is fixed, the size of tLC (57) is decreased.
If tLC (57) is decreased and the actual response time is longer than the assigned response time of the liquid crystal, before the assigned liquid crystal is not yet arranged completely, the backlight emits light and the screen colors are distributed nonuniformly.
FIG. 6 is a flowchart illustrating one frame unit color image display of a field sequential LCD device according to a related art. In the field sequential LCD device, the color image display method sets one frame time to be 1/60 second and turns on and off the R, G, B three color light sources of the R, G, B backlight for 1/180 second (=5.5 msec) for 1/60 second sequentially. At this point, the time that the R, G, B light sources are actually turned on in one frame is shorter than 1/180 second. This is because colors can interfere among red, green and blue if an image is reproduced in a state that the R, G, B light sources are turned on continuously.
As shown in FIG. 6, in the field sequential LCD device, the display of a color image is performed in a sequence of constructing three sub-frames s1, s2 and s3 corresponding to R, G, B color for one frame F that is a basic unit of the screen, turning on and off each of R, G, B light sources 60a, 60b and 60c of a backlight unit sequentially by the interval of 1/180 second, and supplying the liquid crystal panel 61 with the light to display a color image.
But, the field sequential driving method as discussed above is difficult to drive in one frame since the response speed of the liquid crystal is slow. To solve this problem, a divided display area method (DDAM) is used in which a display area is divided into several regions to drive an LCD device.
Next, the backlight unit of a general LCD device driven in the DDAM will be described by referring to FIG. 7.
As shown in FIG. 7, in the general LCD device driven in the DDAM, LED light sources 72 as a backlight unit are disposed on the two opposite sides of a light guide plate 71 configured on the rear surface of the liquid crystal panel (not shown). The liquid crystal panel is lit by the LED light sources 72 so that an image can be displayed in dark place.
Here, each LED light source 72 includes LED lamps 73 arranged in one dimension. The LED lamps 73 are arranged sequentially with red LED, green LED and blue LED on a PCB.
Here, the light guide plate 71 is divided into four regions so as to divide the liquid crystal screen into four regions. The four regions are first to fourth regions 71a, 71b, 71c and 71d and the divided four regions of the liquid crystal screen are drive sequentially. Here, the light guide plate 71 is not physically separated but is defined to be imaginarily divided into four regions.
The LED lamps 73 are turned on by applying a voltage according to the divided regions of the light guide plate 71. The turned-on red, green and blue lights are scattered so that the rear surface of the liquid crystal panel is sequentially lit.
As described above, the LED lamps 73 of each LED light source 72 are turned on sequentially such that only the LED lamp(s) 73 corresponding to a particularly divided region of the plate 71 are driven at a given time to display an image on the liquid crystal panel field sequentially.
However, when only the LED lamps 73 corresponding to a particular divided region of the plate 71 are turned on and driven (when driven in DDAMO), there is generated a light leakage phenomenon in that light is leaked to a neighboring divided region of the light guide plate and liquid crystal panel other than the driving region. Such light leakage deteriorates the display performance of the LCD device.